The present invention relates to a high accuracy positive displacement gas flow meter with horizontal piston movement. The flow meter may advantageously have a low mass piston, constructed of a rigid material such as glass or graphite. Advantageously, a low friction film outer coating of wear resistant material such as diamond-like carbon (DLC) may be formed on the piston.
Positive displacement piston flow meter technology is well established. A piston is fitted precisely into a cylinder with a clearance in the range of up to about 10 microns. Flow introduced into the cylinder displaces the piston. The time required to displace the piston a known distance is used to measure the flow rate. See for example U.S. Pat. No. 5,440,925, incorporated by reference. Current flow meters of this type have pistons constructed of various materials such as glass or graphite. The piston is fitted into a cylinder, commonly made of glass, which is typically oriented vertically to minimize piston wear.
However, in these positive displacement piston flow meters, measurement errors can be introduced from the acceleration of the piston compressing the fluid under measurement as a measurement is initiated; friction between the cylinder and piston causing pressure variations as the flow is being measured; and clearance between the piston and cylinder resulting in a small leakage of fluid through this clearance. This leakage can be measured and added back into the flow measurement. However, minimizing the leakage improves flow measurement accuracy, especially when the fluids to be measured may have different physical properties.